An MS/MS analysis (also called the “tandem analysis”), which is one of the mass spectrometric techniques, has been widely used in recent years, mainly for the purpose of identifying substances having high molecular weights and analyzing their structures. A triple quadrupole mass spectrometer (also called the “tandem quadrupole mass spectrometer” or otherwise) is one type of mass spectrometer capable of MS/MS analyses and is popularly used since it has a comparatively simple structure and is inexpensive.
A triple quadrupole mass spectrometer normally has a collision cell for dissociating an ion by collision-induced dissociation, which is placed between the two quadrupole mass filters provided on the front and rear sides of the cell, respectively. The front quadrupole mass filter selects a precursor ion having a specific mass-to-charge ratio from among various ions derived from a target compound, while the rear quadrupole mass filter separates various product ions produced from the precursor ion according to their mass-to-charge ratios. The collision cell is a box-like structure which is hermetically sealed to a comparatively high degree, into which an inert gas (such as argon or nitrogen) is introduced as the collision gas. The precursor ion selected by the front quadrupole mass filter is given an appropriate amount of collision energy and introduced into the collision cell. Within this collision cell, the ion collides with the collision gas and undergoes the collision-induced dissociation process, whereby the product ions are produced.
The dissociation efficiency of the ion within the collision cell depends on the amount of collision energy possessed by the ion introduced into the collision cell, the pressure of the collision gas in the collision cell (hereinafter, the term “collision-gas pressure” should mean “the pressure of the collision gas in the collision cell” unless otherwise noted), and other factors. Therefore, the detection sensitivity of the product ions also depends on the amount of collision energy and the collision-gas pressure. Additionally, even when the collision-gas pressure is the same, the dissociation efficiency of the ion varies depending on the kind of compound (or to be exact, the kind of precursor ion). Therefore, when a multiple reaction monitoring (MRM) measurement mode for selectively detecting a product ion having a specific mass-to-charge ratio generated from a precursor ion having a specific mass-to-charge ratio is performed in order to determine the quantity of a compound, the analysis operator sets the collision-gas pressure at an optimum level at which the detection sensitivity to the product ion originating from the target compound will be at its highest level.
In recent years, a liquid chromatograph mass spectrometer (LCMS) or gas chromatograph mass spectrometer (GC/MS) in which a liquid chromatograph (LC) or gas chromatogram (GC) is combined with a mass spectrometer has been often used to perform a simultaneous multicomponent analysis for a sample containing a number of compounds (see Patent Literature 1). When a simultaneous multicomponent analysis by an MRM measurement is conducted in an LC/MS or GC/MS, the collision-gas pressure is normally set at a level where the detection sensitivity to a number of target compounds will be high on average. This is not merely because the task of setting the collision-gas pressure at an optimum collision-gas pressure for each compound is troublesome when there are a large number of compounds to be analyzed. Another reason is that it is difficult to completely separate all compounds by the liquid chromatogram or gas chromatograph; in some cases, the elution times of two or more compounds overlap each other, making the determination of the optimum collision-gas pressure complicated and difficult.
For the previously described reasons, when a simultaneous multicomponent analysis is performed by an MRM measurement using a conventional LC/MS or GC/MS, the collision-gas pressure for each compound is not always set at an optimum level in terms of the detection sensitivity. This has been one factor that deteriorates the quantitative accuracy in the simultaneous multicomponent analysis.
Furthermore, to conduct an MRM measurement with the collision-gas pressure set at an optimum level for a target compound in the previously described manner, the analysis operator needs to previously investigate the optimum collision-gas pressure for that compound. For this purpose, the analysis operator must prepare a plurality of analysis condition files with the collision-gas pressure set at a plurality of different levels, perform the MRM measurement a plurality of times according to those analysis condition files, and compare the obtained results, or to perform the MRM measurement for the target compound while manually changing the collision-gas pressure so as to locate the optimum level of the collision-gas pressure. In any case, the task is significantly cumbersome. In particular, a considerable amount of time is needed to find the optimum collision-gas pressures for a large number of compounds.